Here’s my take on an important factor to consider, which I haven’t seen mentioned, in trying to answer the Fermi Paradox (if they are out there, then why haven’t we “seen” them?).
First off, both civilisations need to be able to detect very faint signals, and also transmit very powerful signals. We’ve been in this position for say 50 years, let’s be really generous and say we have another 450 years in front of us before we do ourselves in – then we have a tiny 500 year time-slot over which we can transmit and receive signals – and the aliens have to also be transmitting and receiving data DURING THAT SAME 500 year time slot. So if (for example) they are on a galaxy 1 million light years away – then they will have needed to be transmitting and receiving data over that 500 year period, some 1 million years ago. Seeing as the full time period we should be considering is some 12 billion years, AND including the vast distances involved, it is not too surprising perhaps, that the overlap time (the time in which we could detect each others presence) isn’t too far from zero.
So another possible answer to the Fermi Paradox is that there have been plenty of advanced civilisations out there, and there will be plenty more in the future, but we’ve never shared the same technological time slot.
And add onto that, that you have to be also looking in the right place at the right time, and it makes it look even more unlikely.